1. Field of the Invention
The present invention relates to signal demodulation circuits, and in particular, to quadrature signal demodulators for use in phased array applications, including ultrasound.
2. Related Art
Ultrasound or SONAR pressure waves are generated by feeding a radio frequency (RF) transmit signal voltage to groups of piezoelectric elements in an array. These elements convert the electrical energy to acoustic energy. Such energy conversion is reciprocal in nature, since the same elements, as well as additional elements, convert reflected acoustic energy into electrical energy. Generally, this converted reflected energy is then amplified by a sensitive signal receiver. If the timing or phase of the individual elements is somehow adjusted to form a transmit and/or receive “lens”, the process is called a “phased array” and is used in phased array ultrasound, phased array sonar and phased array radar applications. When circuitry, e.g., digital signal processors (DSPs) create, transmit, focus, steer and detect with the reflected energy beams, the process is called “digital beamforming”.
Doppler frequency shift in ultrasound or sonar is used to determine the velocity of a target relative to the observer. In medical ultrasound applications, the target is usually a moving bolus or volume of blood. If blood flows exactly toward the transmitted beam, the receive signal is shifted up in frequency (positive Doppler shift) by an amount proportional to the product of twice the directed target velocity and the transmitted signal frequency, divided by the velocity of sound in blood. If the directed or effective target velocity is not exactly toward the transmitted signal beam, the frequency shift is multiplied by the cosine of the angle between the instantaneous target velocity vector and that of the transmitted signal beam. Similarly, Doppler shift is negative (frequency decrease) if the directed target velocity is away from the transmitted signal beam. This can be expressed as follows:Δf=(2Vt cos Φ)(fTX)/c 
Δf=Doppler frequency shift
Vt cos Φ=directed target velocity including the angular component
fTX=transmit frequency
c=velocity of sound in blood
Blood is predominantly water, and the velocity of sound in water is approximately 1,580 meters per second (m/sec.) or 1.58 millimeters per microsecond (mm/usec). Assuming the operating frequency, i.e., the transmitted signal frequency, is three megahertz (3 MHz) and normal blood flow in the carotid arteries of the neck varies between 30 and 80 centimeters per second (cm/sec.) throughout the cardiac cycle, and assuming the blood flow is directed 45 degrees from the transmitted signal beam, the resultant Doppler shift, based upon the equation above, will vary between 800 and 2140 Hertz, both of which are well within the audible signal range. When the carotid cross-section is partially occluded by plaque, the blood velocity increases to maintain flow and Doppler shift increases. An unusual shift will be audible to the sonographer and visible when displayed on a color monitor. If the obstruction creates turbulence or cavitation, the effect will be even more noticeable.